Vector modulators are used to transfer a digital quadrature baseband signal to a radio-frequency carrier. The modulated radio-frequency output signal is formed by the addition of the output signal of two doubly balanced mixers that are driven by a quadrature carrier signal.
The power efficiency of known IQ modulators or vector modulators is low since so-called common-mode signals are inherently generated during the modulation, which signals can be suppressed during the conversion of the differential signal into a single-ended signal by an RF balun at the output. The common-mode signals consume current and do not contribute to the information content of the modulated signal. In this respect, FIG. 9 shows an output signal of a vector modulator with the baseband signal I=Q, in which the common-mode signals are marked.
By way of example, 3 dB more output power can be achieved in the case of a phase offset of 90° of the modulated signal with a polar modulator for the same current consumption.
The common-mode signals also generate voltage dips on the supply voltage. The voltage dips modulate the DCO frequency (DCO=digitally controlled oscillator) of the transmitter and lead via the second harmonic to an interference signal in the output spectrum with the frequency w_lo±w_m (where w_lo is the carrier frequency and w_m is the modulation frequency).
In digital vector modulators, the operation of mixing the carrier signal with the digital baseband signal takes place in the digital driving system. The binary output word of the digital part switches a mixer array with switchable current sources. The problem of the common-mode signals furthermore also exists in digital vector modulators.
Furthermore, vector modulators have the disadvantage that the quadrature mixing in known digital vector modulators necessitates two DA converters/mixers or cell arrays (typically one cell array for an in-phase component of a baseband signal and a further cell array for a quadrature or quadrature phase component of the baseband signal). Therefore, the power efficiency is relatively low because the output power is increased only by 3 dB owing to the quadrature mixing in the case of a doubling of the output current.